Now that Disney owns Marvel that would be interesting to worlds based on Iron Man, Captain America, Guardians of the Galaxy, Big Hero 6, Thor and Avengers.
Loki would be the game's main villain as he would visit some of the Disney worlds like Frozen world, Muppet World or Oz world (based on Return to Oz and Oz Great and Powerful) and even speak with other bad guys like Oz's Nome King to even Tex Richman to warn them about Sora and the gang.
Melekith, Mandarin, Whiplash, Thanos, Red Skull and Levithans would make great bad guys to fight and you can summon Hulk, War Machine and Rocket Raccoon as those would be sweet.
Sora and his friends can team up with Iron Man, Thor, Captain America and others.
The final world for the game would be New York City which would consist of 4 levels like Brooklyn (Enchanted), Central Park (Oliver and Company), Manhattan (Gargoyles) and finally Bronx (The Avengers) with Loki in the final level freed from his prison as he uses the power of the heartless to take over the city as you join forces with Hawkeye to destroy the master of the Heartless including Loki himself.